5. SHELL EXPANSION: COLLECT AND COLLAPSE

Zavagno et
al. (2006)
studied star formation in the Milky Way source W 79
(Fig. 4). It has a 1.7 Myr old shell with
gravitationally collapsed regions 0.1 Myr old along the perimeter. This
is an example of star formation triggering by the gravitational collapse
of swept-up gas around an older cluster or OB association. Sh2-219 is a
similar region
(Deharveng et
al. 2006).
There is O9.5V star in a centralized HII
region, and a CO cloud, K-band embedded cluster, Ultra-compact HII
region, and Herbig Be star at the edge.

Figure 4. Milky Way region W 79, consisting
of a shell with dense clouds and star formation at the edge, from
Zavagno et
al. (2006).

Deharveng et
al. (2010)
recently studied 102 bubbles and triggered star
formation using the Spitzer-GLIMPSE and MIPSGAL surveys for the IR, the
MAGPIS and VGPS surveys for the radio continuum, and the ATLASGAL
survey at 870 µm for cold dust emission. They found that 86% of
the bubbles contain HII regions, and among those with adequate
resolution, 40% have cold dust along their borders, presumably
accumulated during the bubbles' expansions. Eighteen bubbles have
either ultracompact HII regions or methanol masers in the peripheral
dust, indicating triggering. They categorized their results into
several types of triggering. Star formation that occurs in pre-existing
cloud condensations is distinctive because the clouds protrude into the
bubble cavity like bright rims; 28% of the resolvable shells are like
this. Star formation that occurs by the collapse of swept-up gas does
not protrude but is fully in the shell. That is because it is comoving
with the shell. In fact, clumps forming by gravitational collapse in a
shell could eventually protrude out of the front of the shell because
their higher column densities makes them decelerate slower than the
rest of the shell
(Elmegreen 1989).
If this is observed, then the relative
position of a triggered clump and the shell around it should indicate
their relative speeds and the time when the clump first formed.

Beerer (2010)
studied Cygnus X North with Spitzer IRAC, classifying
stars according to their IR spectral ages. They found that younger
objects are in filaments that look compressed. Triggering of those
stars was suggested.

Desai (2010)
examined all 45 known supernova remnants in the LMC and
looked for associations with young stellar objects and with GMCs that
have no YSOs. Seven SNR were associated with GMCs and YSOs, 3 SNRs
were with YSOs and no GMCs, and 8 SNRs were with GMCs and no YSOs. For
the 10 SNRs with YSOs, only 2 have YSOs that are clearly associated
with the SN shell, but in these cases, the SNe are younger than YSOs,
so the YSOs could not have been triggered. Desai et al. concluded that
SNe are too short-lived for triggering.